- Our login page asks for "username" but we expect you to provide an email as your username. People keep clicking "Forgot username", so we should document that your username is the email you signed up with.
- When patching the high-availability system, support must patch the passive half of the pair first. Patching the active half first leads to system downtime. We should document that for support.
- We can register our system in DNS at installation time (go us!) but only if you provide the username and password of someone with permission to do so.
Every "we should document that" is a bug until proven otherwise.
If you have to document it, most of the time that means you've designed it or implemented it in a confusing way. So yes, please document it. Then log it as a bug and make it less confusing. Sometimes that's as simple as changing a label - "email" instead of "username". Sometimes it's harder, like making a high-availability configuration patchable from either half with transparent failover. Sometimes, it's even dependent on a third-party system, and involves moving the documentation into the product (e.g., "please enter the username and password for a user with permission to create DNS entries") rather than removing documentation entirely.
So no, we shouldn't document that. Instead, we should fix that. Let's quit with the post facto workarounds and start actually fixing the weirdness in our application up front.